The 9/11 memorial will preserve traces of the north tower’s steel columns and the building’s bedrock footprint, which visitors will be able to view in an underground room, sources said yesterday.

The revised memorial design, which is being fleshed out this week, would respond to groups of 9/11 family members who have demanded the footprints of both World Trade Center towers be preserved at bedrock.

Sources said visitors to the Reflecting Absence memorial would be able to descend to the bottom of the trade center pit, where they could walk on the one-acre footprint of the north tower – underneath one of the memorial’s two reflecting pools – within a large underground room.

The tower’s massive steel columns were cut down and removed along with hundreds of tons of other WTC debris, but nubs of the steel remain visible, embedded in the bedrock.

The memorial designer, Michael Arad, has also proposed a vertical connection from bedrock to the surface that would let visitors look up from the north tower footprint and see the sky, or possibly water cascading down from the pool above, the sources said.

It’s not clear if the entire footprint will be accessible, since the Port Authority has said a new PATH train platform may cut across one corner. Much of the south footprint at bedrock is already taken up with PATH tracks.

Arad’s memorial centers above ground on two reflecting pools in the tower footprints – but he added the bedrock concept in recent revisions.

The bedrock room at the north footprint – which would be six stories below street level – will connect to another underground space where visitors could view the trade center’s basement slurry wall.

The concrete slurry wall, also known as the bathtub, was a central, symbolic component of the site’s master plan, created by architect Daniel Libeskind.

Arad’s original design left out any mention of the slurry wall and filled in the Ground Zero pit that left the wall exposed.

Arad’s plan also eliminated a museum and other cultural buildings bordering the memorial.